Rechargeable fuel cell and method

ABSTRACT

An apparatus for controlling humidity in a fuel cell is provided. The apparatus can include a housing having an interior surface defining a volume and containing at least one fuel cell disposed within the volume. The apparatus may also include a plurality of electrodes disposed within the volume. The apparatus can include an aperture defined by the housing through which at least one air gas stream can be in fluid communication with at least one of the plurality of electrodes. The apparatus can include a humidity-controlling component, wherein the humidity-controlling component is in fluid communication with the air gas stream prior to the air gas stream contacting the at least one of the plurality of electrodes, and humidity-controlling component being capable of controlling a relative humidity of the air gas stream. A method for controlling humidity in a fuel cell is provided.

BACKGROUND

1. Technical Field

An embodiment of the invention may relate to a fuel cell or secondarybattery. An embodiment of the invention may relate to a methodassociated with a fuel cell or secondary battery.

2. Discussion of Related Art

Water loss constitutes an obstacle to optimal performance ofrechargeable fuel cells and secondary batteries. In a rechargeable fuelcell, metal hydride may be used as the anode and oxygen may be used as acathode. Oxygen is consumed to form hydroxide ions when the fuel cell isdischarging electricity. Thus, there must be a continuous supply ofoxygen during the operation. Conversely, oxygen is generated and ventedwhen the cell is recharged. Therefore, fuel cells are designed as opensystems to permit intake and venting of oxygen. This open design leadsto a water management problem when the ambient humidity is lower thanthe equilibrium humidity of the electrolyte in the cell. The cellgradually dries out, while in the later case, the electrolyte is dilutedor even overflows from the cell. The life span of a cell can bedramatically impaired when an imbalance of water occurs. Conversely,when the relative humidity is higher than about 65 percent, the systemcan gain water from the wet air and flood.

Previously, water loss has been reduced in fuel cells, or metal airbatteries, by restricting the size of openings into the fuel cell.According to this method, the openings may be as small as a few dozenmicrometers in diameter. The restricted opening may reduce waterevaporation, but also restricts gas flow. A fan can be added to move gasand vapor within the fuel cell to overcome the restricted opening size.But, fans reduce efficiency of the fuel cell due to the fan energyconsumption. Another water loss control is through the use of aselective permeation membrane made of a metal oxide, such as tin oxide.The pore size in the metal oxide membrane is too small to allow the flowof enough oxygen. This limits the power of such cells. Another approachis to add a hygroscopic chemical, such as KOH solution. The increasedhydrogen bonding and the increased viscosity results in a reduction inthe rate of water loss. However, this is still not a satisfactorysolution since the cell can still dry out over time.

It may be desirable to have a device that is able to create and/ormaintain a stable humidity inside a rechargeable fuel cell. Furthermore,it may be desirable to have a method for creating and/or maintaining astable humidity inside a rechargeable fuel cell.

BRIEF DESCRIPTION

The invention may include embodiments that may relate to a devicecapable of creating and/or maintaining a stable humidity inside arechargeable fuel cell or battery. The invention may also includeembodiments that may relate to a method of creating and/or maintaining astable humidity inside a rechargeable fuel cell or battery.

According to one embodiment, an apparatus is provided that includes ahousing having an interior surface. The interior surface defines avolume and contains a plurality of electrodes. An aperture is defined bythe housing, and through which an air gas stream can be in fluidcommunication with at least one of the plurality of electrodes. Ahumidity-controlling component is in fluid communication with the airgas stream prior to the air gas stream contacting the at least one ofthe plurality of electrodes, and the humidity-controlling component cancontrol a relative humidity of the air gas stream.

A method includes passing an intake air gas stream over a humiditybuffer solution. The humidity of the at least one intake air gas streamis adjusted to about the equilibrium humidity relative to the humiditybuffer solution. The method includes providing the humidity-adjustedstream to at least one electrode in a fuel cell.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a graphic plot showing a relationship between relativehumidity and temperature according to one embodiment;

FIG. 2( a) is a drawing of an embodiment having a tray for containing ahumidity buffer solution;

FIG. 2( b) is a graphic plot comparing ambient humidity, the humidityover pure water and the humidity over saturated NaC solution;

FIG. 3 is a graph having a pair of plots showing voltage (top) andcurrent (bottom) discharge profiles of a cell according to an embodimentof the invention;

FIG. 4 is a graph having a pair of plots showing voltage (top) andcurrent (bottom) discharge profiles of a cell according to an embodimentof the invention;

FIG. 5 is an exploded schematic view of an embodiment having a thirdelectrode;

FIG. 6 is an exploded schematic view of another embodiment having athird electrode;

FIG. 7 is a graphic plot showing the effect of the third electrode ondischarge capacity;

FIG. 8 is a flowchart showing a process for making a third electrodehaving a frame structure; and

FIG. 9 is a flowchart showing a process for making a galvanicelectrochemical cell having a third electrode.

DETAILED DESCRIPTION

The invention may include embodiments that relate to a fuel cell orsecondary battery. In one embodiment, a method associated with the fuelcell or secondary battery is provided.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termsuch as “about” may not to be limited to the precise value specified,and may include values that differ from the specified value. In at leastsome instances, the approximating language may correspond to theprecision of an instrument for measuring the value. Similarly, “free”may be used in combination with a term, and may include an insubstantialnumber, or trace amounts, while still being considered free of themodified term.

As used herein the term humidity buffer solution includes a compositionof matter that is capable of absorbing excess water from, or addingreplacement water to, an electrolyte in contact therewith. Thiscapability may include producing and maintaining an equilibrium humidityat or near that of the selected electrolyte solution. Some humiditybuffer solutions may comprise aqueous solutions of one or more organicor inorganic salts. Furthermore, the solvent is not limited to water.Water may be combined with one or more soluble or semi-solubleadditives.

As used herein, the term membrane refers to a selective barrier thatpermits passage of protons and/or hydroxide ions generated at a cathodethrough the membrane to the anode for oxidation of hydrogen atoms at theanode to form water and heat, unless context or text indicatesotherwise.

One embodiment may comprise a tray for holding a humidity buffersolution. The tray may be disposed within a housing with a fuel cellelectrolyte solution and can be spaced apart from the fuel cellelectrolyte solution. The space or volume between the fuel cellelectrolyte solution and the humidity buffer solution may be occupied bya gas phase. According to this embodiment, the gas phase is in contactwith both the fuel cell electrolyte solution and the humidity buffersolution. Therefore, water may pass from the humidity buffer solution tothe fuel cell electrolyte solution, or from the fuel cell electrolytesolution to the humidity buffer solution. Accordingly, as the fuel cellelectrolyte solution loses water, the losses can be compensated bydrawing replacement water from the surrounding gas phase. Furthermore,the surrounding gas phase may maintain a substantially constant relativehumidity because it may draw replacement water from the humidity buffersolution. Conversely, if the fuel cell collects, absorbs, or createsexcess water, the excess can be expelled from the electrolyte solutionby vaporizing it into the surrounding gas phase. Furthermore, thesurrounding gas phase may maintain a substantially constant relativehumidity because it may release water to the humidity

Humidity control materials can provide a stable humidity that can besuitable for rechargeable fuel cells. Suitable humidity controlmaterials include saturated solutions of organic or inorganic salts,drying agent solutions, polymer gels, and inorganic colloids. Suitablehumidity buffer solutions can comprise one or more alkaline earth metalsalt, which may be a halide, sulfate, carbonate, nitrate, or caboxylate.Suitable salt solutions may include one or more of CaSO₄, LiCl, CH₃COOK,MgCl₂, KCO₃, Mg (NO₃)₂, NaBr, CoCl₂, NaNO₂, SrCl₂, NaNO₃, NaCl, KBr,(NH₄)₂SO₄, KCl, Sr(NO₃)₂, BaCl₂, KNO₃, or K₂SO₄. The humidity buffersolution may be a saturated solution. In one embodiment, the saturatedsolution may consist essentially of one or more of CaSO₄, LiCl, CH₃COOK,MgCl₂, KCO₃, Mg (NO₃)₂, NaBr, CoCl₂, NaNO₂, SrCl₂, NaNO₃, NaCl, KBr,(NH₄)₂SO₄, KCl, Sr(NO₃)₂, BaCl₂, KNO₃, or K₂SO₄. Table 1 sets forth aplurality of humidity buffer solutions that can provide equilibriumhumidities in a suitable range of humidities.

TABLE 1 Equilibrium humidity of saturated salt solutions SALT 25° C. 30°C. CaSO₄ <0.01 <0.01 LiCl 0.112 0.115 CH₃COOK 0.227 0.225 MgCl₂ 0.3280.329 KCO₃ 0.432 0.447 Mg (NO₃)₂ 0.529 0.52 NaBr 0.576 0.574 CoCl₂ 0.649— NaNO₂ 0.643 0.649 SrCl₂ 0.709 — NaNO₃ 0.743 — NaCl 0.753 0.769 KBr0.809 — (NH₄)₂SO₄ 0.81 — KCl 0.843 0.85 Sr(NO₃)₂ 0.851 — BaCl₂ 0.9020.92 KNO₃ 0.936 — K₂SO₄ 0.973 0.977

These compositions may produce equilibrium humidities during use thatare in a range of from about 50 percent to 65 percent, 65 percent toabout 75 percent, or 75 percent to about 90 percent of the equilibriumhumidity of 6M KOH. Such substances can generate a local environmenthaving a humidity that is stable and suitable for maintaining waterbalance in a rechargeable fuel cell.

FIG. 2 shows a fuel cell device equipped with a tray or well for holdinga humidity buffer solution. The tray can contain any of a variety ofsaturated salt solutions. According to this embodiment, the traycomprises a part of a fuel cell stack. One side of the cell faces thesurface of the humidity buffer solution. The humidity buffer solutionprovides a stable humidity at or near the equilibrium humidity of theelectrolyte.

The humidity buffer solution may further include a hydrophilic additive.Suitable hydrophilic additives may include a polyacrylate, for example,sodium polyacrylate (PAA Na) CAS#: 9003-04-7. Additionally oralternatively, other suitable hydrophilic additives may include one ormore alcohols, amines, ethers, or cellulosics. Suitable alcohols may bepolyols, such as polyethylene glycol. In one embodiment, the hydrophilicadditive may include one or more of glycerin, carboxymethyl cellulose(CMC), or polyethylene oxide. In one embodiment, the hydrophilicadditive may include one or more of polyacrylamide, polyvinyl alcohol orpoly(vinyl acetate). The hydrophilic additives may include one or morefunctional groups that are effective for bonding with water. Suitablefunctional groups may include one or more of OH—, carboxyl, ether, andNH— functional groups. In one embodiment, more than one type offunctional group is present on a single molecule.

The PAA Na, glycerin, polyethylene oxide, carboxymethyl cellulose (CMC),alcohols and amine additives may be soluble in water. The chemicalformula for PAA Na is:

—[CH₂—CH(COONa)]_(n).

The chemical formula for carboxymethyl cellulose (CMC) is:

Suitable hydrophilic additives may have a molecular weight of up toabout 3,000,000. In one embodiment, the hydrophilic additive averagemolecular weight may be in a range of from about 50,000 to about500,000; from about 500,000 to about 750,000; from about 750,000 toabout 1,000,000; from about 1,000,000 to about 1,500,000; from about1,500,000 to about 2,000,000; from about 2,000,000 to about 2,500,000;from about 2,500,000 to about 2,750,000; or from about 2,750,000 toabout 3,000,000.

The hydrophilic additive may be present in the humidity buffer solutionin a concentration effective for reducing water evaporation from theelectrochemical cell. The hydrophilic additives may be present in thehumidity buffer solution in an amount of up to about 95 weight percentbased on the weight of the humidity buffer solution. In one embodiment,the hydrophilic additive may be present in the humidity buffer solutionin an amount in a range of from about 0.5 weight percent to about 1.5weight percent, from about 1.5 weight percent to about 2.5 weightpercent, from about 2.5 weight percent to about 5 weight percent, fromabout 5 weight percent to about 7.5 weight percent, from about 7.5weight percent to about 15 weight percent, from about 15 weight percentto about 25 weight percent, from about 25 weight percent to about 50weight percent, from about 50 weight percent to about 65 weight percent,from about 65 weight percent to about 80 weight percent, or from about80 weight percent to about 95 weight percent based on the weight of thehumidity buffer solution.

During use, the hydrophilic additives may absorb water vapor from airand may retain the water in the humidity buffer solution. The presenceof the hydrophilic additives in the humidity buffer solution may reducethe equilibrium vapor pressure of the humidity buffer solution. Arelatively lower equilibrium vapor pressure may retain relatively morewater in the humidity buffer solution as liquid.

In one embodiment, the humidity buffer solution is potassium hydroxide(KOH) and has a molarity of 6 mol/L. The KOH humidity buffer solutionmay be mixed with PAA Na to form a KOH/PAA Na/water solution. Thesolution of KOH/PAA Na/water may absorb water as water vapor fromambient air into the humidity buffer solution, and may retain that waterwithin the electrochemical cell. This absorption of water from watervapor by PAA Na in the humidity buffer solution may results in a netwater retention even under conditions where the relative humidity of thevapor environment in the electrochemical cell is reduced because theequilibrium of the system favors retention of water in the humiditybuffer solution.

The KOH/PAA Na/water humidity buffer solution may maintain water becausewhen water evaporation increases, forming more water vapor, the KOH andPAA Na concentrations also increase within the humidity buffer solution.As a consequence, evaporation of water from the humidity buffer solutionis decreased because the equilibrium vapor pressure for water favorsretention of water in the humidity buffer solution. The waterconcentration increase in the humidity buffer solution continues untilthe vapor pressure favors water evaporation. This self-regulatingwater/water vapor dynamic may reduce or prevent a risk of theelectrochemical cell drying out. This aspect may maintain a waterbalance in the cell within a determined range. For some embodiments, thePAA Na showed such effect up to about 800 times its weight in water.

While a KOH/PAA Na/water humidity buffer solution has been described, itis understood by one of ordinary skill in the art that other hygroscopicadditives, such as alcohols, amines and glycerin are usable within anaqueous humidity buffer solution.

According to some embodiments, a fuel cell component may derive hydrogenfrom a solid-state material and water, or from another hydrogen source.A porous metal hydride anode of the fuel cell may be operable forconducting electrons freed from the solid-state hydrogen storagematerial so that they can be supplied to current collectors. The porousmetal hydride anode may include pores, and interstitial spaces that areoperable for storing water and electrolyte. The porous metal hydrideanode may have an improved charge efficiency occurring as a result ofreducing electrolyte transfer. According to some embodiments, porositymay create a volume within the anode for storage of water and/orelectrolyte, which may be effective for off-setting water losses due toevaporation and consumption. For example, water may be retained in aporous metal hydride anode fabricated using sintered zinc powder.

The terms cathode and cathodic electrode refer to an electrode this ispositively charged during a discharge operation. At the cathode, orcathodic electrode, oxygen from air is reduced by free electrons fromthe usable electric current, generated at the anode, that combine withwater, generated by the anode, to form hydroxide ions and heat. A fuelcell cathode can conduct electrons back from an external circuit to acatalyst, where they combine with water and oxygen to form hydroxideions. The catalyst may be operable for facilitating the reaction betweenhydrogen and oxygen. The catalyst may comprise materials including, butnot limited to, platinum, palladium and ruthenium, which face theseparator membrane. The surface of the platinum may be such that amaximum amount of the surface area may be exposed to oxygen. Oxygenmolecules are dissociated into oxygen atoms in the presence of thecatalyst and accept electrons from the external circuit while reactingwith hydrogen atoms, thus forming water. In this electrochemicalreaction, a potential develops between the two electrodes.

A hydrogen-generating component of a hybrid system provides energystorage capacity and shares the porous anodic electrode of the fuel cellcomponent. The hydrogen-generating component further may include anelectrode and a separator membrane. The structure of thehydrogen-generating component may be a construction including one ormore identical cells, with each cell including at least one each of anelectrode, anodic electrode, and separator membrane. The anodic porouselectrode may include a hydrogen storage material and may perform one ormore functions, such as: (1) a solid-state hydrogen source for the fuelcell component; (2) an active electrode for the hydrogen-generatingcomponent; and (3) a portion or all of the electrode functions as ananode of the anode component.

The electrochemical hydrogen-generating component has storagecharacteristics characterized by being capable of acceptingdirect-current (DC) electrical energy in a charging phase to return thesolid-state material to a hydrogen-rich form, retaining the energy inthe form of chemical energy in the charge retention phase, and releasingstored energy upon a demand by the fuel cell component in a dischargephase. The hydrogen-generating component may repeatedly perform thesethree phases over a reasonable life cycle based on its rechargeableproperties. The electrical energy may be supplied from an externalsource, a regenerative braking system, as well as any other sourcecapable of supplying electrical energy. The solid state material may berecharged with hydrogen by applying the external voltage.

Suitable metal hydrides may include one or more of AB₅ alloy, AB₂ alloy,AB alloy, A₂B alloy, A₂B₁₇ alloy, or AB₃ alloy. The AB₅ alloy mayinclude, but is not limited to, LaNi₅, CaNi₅, or MA_(x)B_(y)C_(z),wherein M may be a rare earth element component; A is one of theelements Ni or Co; B may be one of the elements Cu, Fe or Mn; (it isnoted that as used herein “C” does not stand for elemental carbon) C maybe one of the elements Al, Cr, Si, Ti, V or Sn. And, x, y and z satisfyone or more of the following relations, wherein 2.2≦x≦4.8, 0.01≦y≦2.0,0.01≦z≦0.6, or 4.8≦x+y+z≦5.4. Suitable examples of AB₂ type alloysinclude, but are not limited to, Zr—13 V—Ni, Zr—Mn—Ni, Zr—Cr—Ni, TiMn,and TiCr. Suitable AB type alloys include, but are not limited to, TiFeand TiNi. Suitable A₂B type alloys include, but are not limited to,Mg₂Ni. Suitable A₂B₁₇ type alloys include, but are not limited to,La₂Mg₁₇. Suitable AB₃ type alloys include, but are not limited to,LaNi₃, CaNi₃, and LaMg₂Ni₉.

In one embodiment, the anode material may include catalyzed complexhydrides. Suitable complex hydrides may include one or more of borides,carbides, nitrides, aluminides, or silicides. Suitable examples ofcomplex catalyzed hydrides may include an alanate. Suitable alanates mayinclude one or more of NaAlH₄, Zn(AlH₄)₂, LiAlH₄ and Ga(AlH₄)₃. Suitableborohydrides may include one or more of Mg(BH₄)₂, Mn(BH₄)₂ or Zn(BH₄)₂.In one embodiment, the anode material may include complex carbon-basedstructures or boron-based structures. Such complex carbon-basedstructures may include fullerenes, nanotubes, and the like. Such complexboron-based structures may include boron nitride (BN) nanotubes, and thelike.

Sacrificial additives may be selected to control the pore volume and/orthe pore configuration. For example, a weight of sacrificial additivesmay be selected to control pore volume. That is, the more of thesacrificial additive used, the more pore volume is generated when thesacrificial additive is removed. As another example, a type ofsacrificial additive may be selected to control pore configuration. Thatis, the configuration of the sacrificial additive selected may controlthe pore configuration when the sacrificial additive is removed. Theconfiguration may include such attributes as interconnectivity,diameter, length, spacing, and the like.

In one method embodiment, metal hydride powder may be mixed with aconductive additive. Suitable conductive additives may include, forexample, nickel or cobalt.

A determined amount of sacrificial additives may be added to form amixture. The amount may be determined with reference to the desired porevolume of the end product. That is, an amount of the sacrificialadditives having a known volume may be used to produce a correspondingdesired volume in the end product. Suitable sacrificial additives mayinclude one or more of zinc, aluminum, nickel, or carbon. In oneembodiment, the sacrificial additives may include one or more of zincacetate, aluminum acetate, or nickel acetate. In one embodiment, thesacrificial additives may include a carbonate, such as NH₄HCO₃.

The mixture may be pasted, formed, and/or pressed to form an anodeelectrode precursor structure. The anode electrode precursor structuremay be heated. The heating may calcine and/or sinter the precursorstructure to form an electrode main body. The sacrificial additives maybe partially or entirely removed during, or after, the sintering and/orcalcining process. If removed during, the heat of calcining and/orsintering may vaporize the sacrificial additives. If removed after, thesacrificial additives may be solvated or the like. Excipient salts maybe useful for solvated removal after heating. The removal of thesacrificial additives may leave a porous metal anode electrode main bodyhaving a determined pore volume.

In one embodiment, the sacrificial additive may be selected to have aneffect on the inner surface of the pores formed by the removal of thesacrificial additive. In such an instance, the composition of thesacrificial additive may be entirely or partially devoted to affectingthe surface character of the pore. For example, if a metal particle isadded to the sacrificial additive, which is otherwise a volatile lowpolymer, heating to vaporize the sacrificial additive may release themetal particle from the matrix of the sacrificial additive and the metalparticle may deposit on the pore inner surface. Thus, the pore innersurface composition and character may be controlled. In one embodiment,a material is deposited on the pore inner surface that readily formssurface hydroxyl groups. The surface hydroxyls may increase thehydrophilicity of the pores and facilitate transport of polar liquidstherethrough. In one embodiment, a selected catalyst may be deposited onthe pore inner surface. It may be desirable to coat the outer surface ofthe sacrificial additive, which will contact and define the innersurface of the pore, with the material to be deposited.

During use, the pores may receive and store water and/or electrolyte.Suitable electrolytes may include aqueous KOH. The anode electrode mainbody may have a pore volume capable of storing quantities of water andelectrolyte suitable for use in a rechargeable fuel cell or a metalhydride based battery. The pore volume may be greater that about 5percent of the volume of the anode electrode main body. In oneembodiment, the pore volume may be in a range of from about 5 percent toabout 10 percent, from about 10 percent to about 15 percent, from about15 percent to about 20 percent, from about 20 percent to about 25percent, from about 25 percent to about 35 percent, from about 35percent to about 45 percent, from about 45 percent to about 55 percent,or from about 55 percent to about 75 percent of the volume of the anodeelectrode main body.

Embodiments of the porous metal hydride anode may have a relativelyimproved charge efficiency resulting from a reduced electrolytetransfer. Electrolyte transfer may refer to the tendency of theelectrolyte to migrate from the positive end proximate the cathode tothe negative end proximate the anode during use. In a stack,particularly, the end cells may lose performance relative to thecentrally located cells due to such migration, which may cause aconcentration imbalance. By providing a physical obstacle to flow, inthe form of a tortuous path and constricted pathways, electrolytemigration may be controlled, and thereby electrolyte transfer may bereduced.

Thus, some porous metal hydride electrode embodiments can storeadditional KOH electrolyte and can serve as anodes after beingpositioned with a membrane separator, air cathode electrode and othercomponents and assembled into a rechargeable fuel cell. The additionalquantity of KOH electrolyte stored in porous anode embodiments canreduce the water management concerns caused by the consumption andevaporation of water during the charge and discharge processes. At thesame time, the use of porous anode embodiments in a rechargeable fuelcell improves the energy conversion and energy transfer efficiency ofthe fuel cell. The porous anode is also usable in fuel cells that arenot rechargeable.

In one aspect, an embodiment may include a method for making a porousanode for use in a rechargeable fuel cell. The method may include,preparing a mixture. The mixture may include metal hydride and one ormore sacrificial additives. For some embodiments, a gel binder may beadded as part of the sacrificial additive. The additives may besacrificial insofar as they may be subsequently removed during sinteringand/or calcining, completely or in part, to form the pores of the porousanode.

The metal hydride and sacrificial additive mixture may be formed into aporous electrode main body, or green body. The green body may besintered. Sintering may obtain a stable and strong connection among themetal hydride particles. Hydrogen gas may be introduced during sinteringto reduce or prevent metal hydride oxidation. The sacrificial additivemay be introduced during mixing, and may be removed during sinteringand/or calcining. Alternatively, the sacrificial additive may be removedby other removal steps without sintering.

The metal hydride and sacrificial additive mixture may be pastesintered. In this paste-sintered embodiment, a mixture of metal hydrideand sacrificial additive may coat a metal foam plate, and may be pastesintered at a relatively high temperature.

In one embodiment for paste sintering, a nickel metal hydride may bemixed with a zinc sacrificial additive, forming a metal hydride mixture.The metal hydride mixture may be applied to a nickel foam. The wetcoated nickel foam plate may be dried to form an electrode main body.The main body may be sintered at about 800 degrees Celsius. In oneembodiment, the metal hydride mixture may be mixed further with abinder. Suitable binders may include styrene butadiene rubber andnickel. The mixed composition may be cold pressed onto the nickel foamplate to form a cold pressed assembly. The cold press assembly may becold press sintered at a lower temperature than the temperature used forpaste sintering.

The temperature range for paste sintering may be from about 100 degreesCelsius to about 800 degrees Celsius. The temperature range for coldpress sintering may be in a range of from about 100 to about 300 degreesCelsius. Binders such as gel binders, styrene butadiene rubber, andcarboxymethyl cellulose may be added to the cold press assembly and maybe sintered at a temperature in a range of from about 500 degreesCelsius to about 800 degrees Celsius. Sacrificial additives may be addedto the mixture before it is formed green structure, which may be furtherprocessed to become the electrode main body.

The sintered anodes may be treated to remove sacrificial additives. Thetreatment may include sonication, acidification, solvation, ordissolution by heat decomposition. Additive removal schemes for removingadditives in an alkaline environment with sonication include treatingwith zinc or aluminum as follows:

Zn+2OH⁻→ZnO₂ ⁻+H₂

Al+2OH⁻→AlO2⁻+H₂

The treatment with an alkaline material forms Zn and Al ionic species,which may be washed away.

Additive removal schemes for removing additives in an acidic environmentwith sonication may include treating with zinc or aluminum or ammoniumcarbonate as follows:

Zn+2H⁺→Zn²⁺⁺H₂

Al+2H⁺→Al³⁺+H₂

When Zn and Al are exposed to an acidic environment, zinc ion andaluminum ion, respectively, may be formed with hydrogen gas.

In one method embodiment, a sacrificial additive of aluminum powder maybe mixed into anodic metal hydride material to form a mixture. Themixture may be coated onto a nickel foam and pressed to form an anodehaving a thickness of, In one embodiment, about 3 mm. The anode may besoaked in an alkaline solution to remove the aluminum. The soaked anodemay be sintered in a mixture of argon gas and hydrogen gas, for someembodiments. For other embodiments, the anode may not be sintered.

Another method embodiment may include mixing NH₄HCO₃ into anodic metalhydride material, coating the mixture onto nickel foam and pressing toform an anode. In one embodiment, the thickness of an anode may be about3 mm. The pressed anode may be heated at a temperature of about 60degrees Celsius to remove the NH₄HCO₃ with the removal scheme below.

NH₄HCO₃→NH₃+CO₂+H₂O

Another method may include mixing nickel acetate into anodic metalhydride material, coating the mixture onto a nickel foam plate to forman anode. The anode may be heated to about 500 degrees Celsius to removeacetate ions, and form a porous anode with the removal scheme below. Inanother embodiment, the anode may be pressed to form an anode havingthickness of about 3 millimeters (mm). The pressed anode may be heatedto about 500 degrees Celsius to remove acetate ions, for example withthe removal scheme below.

Ni(CH₃COO)_(2 l +H) ₂ 43 Ni+C+CO₂+H₂O

The pore volume of the porous anode may be determined by selecting aquantity of sacrificial additive, such as aluminum and zinc that producethe pore volume. The mechanical strength of the porous anode may bedetermined by selecting the pressure and time of sintering. Thesintering effect may be affected by controlling the temperature and thetime of sintering. The sintering process may destroy or chemically alterthe binders, such as polytetrafluoroethylene and carboxymethylcellulose.

In one embodiment, hydrogen and/or oxygen may be required by a fuel cellcomponent to produce electrical energy. A rechargeable fuel cell may beoperated with solid-state materials capable of hydrogen storage, suchas, but not limited to, conductive polymers, ceramics, metals, metalhydrides, organic hydrides, a binary or other types of binary/ternarycomposites, nanocomposites, carbon nanostructures, hydride slurries andany other advanced composite material having hydrogen storage capacity.Recharging of a rechargeable fuel cell may produce water and/or oxygen,which may be recycled. The electrochemical system may require coolingand management of the exhaust water. The water produced by the fuel cellcomponent may recharge the solid-state fuel. For some embodiments, theonly liquid present in the rechargeable fuel cell may be water orwater-based solutions. Water management in the non-woven separationmembrane may be useful. Because the membrane may function better ifhydrated, the fuel cell component may operate under conditions where thewater by-product does not evaporate faster than it may be produced. Theporous metal anode embodiments described herein may aid in themaintenance of membrane hydration.

The rechargeable fuel cell embodiment described herein applies to powergeneration in general, transportation applications, portable powersources, home and commercial power generation, large power generationand to any other application that would benefit from the use of such asystem.

While a fuel cell/hydrogen generator hybrid design may be shown, it maybe understood that other rechargeable fuel cell embodiments may includethe porous metal hydride anode. The rechargeable fuel cell described maybe operable for converting electrical energy into chemical energy, andchemical energy into electrical energy.

A third electrode may include a material with low oxygen evolutionover-potential. The third electrode may include one or more ferro-basedalloys. Suitable ferro-based alloys may include stainless steel. Otherexamples of suitable materials may include one or more of cadmium,palladium, lead, gold, or platinum. The material may be configured toincrease surface area, such as by foaming. A suitable example would be anickel-based foam. Foams may enhance an ability of storing electrolytesolution within the volume of its pores, may provide an increasedsurface area for reaction, and may provide for diffusion control.

Referring to FIG. 5, an exploded view of an assembly 500 of cathode 507and third electrode assembly 501 utilized in a galvanic cell are shown.The cathode 507 and third electrode assembly 501 may include a cathode507 surrounded on the perimeter by a separator 535. The third electrode503 further surrounds the separator 505 in a single plane with thecathode 507.

The third electrode frame structure 501 may be in contact on a firstside with a second separator 545, which may be in contact with an anode509. The anode 509 may contact both electrodes of the third electrodeframe structure 501, through the second separator 545, despite thecathode 507 and third electrode 503 being electrically insulated fromone another. A sealing ring 517 may be positioned on the perimeter ofthe anode 509. The anode 509 may contact a third separator 555. An airpermeable cover 519 and support cover 513 may structurally support thecell. Water filling ports and electrical connections 515 may beincorporated to complete the cell.

The covers may function as a plastic housing. The plastic housing may bemoldable to reduce cost and to simplify manufacturing. Thecovers/housing 513 and 519 may include polyethylene or polypropylene,for example. Such materials allow for a caustic resistant housing forthe cell and its components. The moldable housing may provide relativelyefficient sealing. In one embodiment, the plastic housing may bethermoset. Thus, the plastic housing may be formed by, for example,resin injection molding (RIM) or from a bulk molding compound (BMC).

Applying a voltage between the anode and the third electrode of the celland reversing the electrochemical reaction may recharge an electricallyrechargeable fuel cell or metal/air battery. During recharging, the cellmay generate oxygen. Generated oxygen may be released to the atmospherethrough the air permeable cathode if desired.

The mechanism of a rechargeable fuel cell or metal/air battery may beshown below.

In charging process:

negative electrode: 4M + 4H₂O + 4e → 4MH + 4OH⁻ third electrode: 4OH⁻ →O2 + 2H₂O + 4e total electrolysis reaction: 4M + 2H₂O → 4MH + O₂In discharging process:

negative electrode: 4MH + 4OH⁻ + 4e → 4M + 4H₂O positive electrode: O₂ +2H₂O + 4e → 4OH⁻ total cell reaction: 4MH + O₂ → 4M + 2H₂O

The cathode may be used during the discharge cycle, but may beinefficient in recharging the cell. Further, the cathode may deterioratequickly when used to recharge. In one embodiment, a third electrode maybe utilized as a separate oxygen generation electrode. According to someembodiments, a third electrode may be utilized to extend the cycle lifeover traditional structures by chemically and mechanically protectingthe cathode from degradation during recharge. The charge process takesplace between the anode and the third electrode. The discharge processtakes place between the anode and the cathode. Therefore, the cathodecan be free from damage during the oxygen evolution reaction.

Referring to FIG. 6, an exploded view of a galvanic cell embodimenthaving a third electrode frame structure is shown. Current collectors621 on the cathode 607 side contact a cathode 607 and a third electrode603 separately. That is, two electrically insulated leads stretch outfrom cathode 607 and frame third 603 separately. Both the thirdelectrode 603 and the cathode 607 may be separated from an anode 609 bya separator 655. FIG. 7 shows a plot of the cycle life of the foregoinggalvanic cell. The galvanic cell may utilize the assembly of the cathodeand the third electrode assembly. The cell may be compared to a cell inthe absence of the third electrode 603. The graph displays the extendedcycle life provided by the third electrode 603.

Referring to FIG. 8, a process for making a third electrode framestructure is shown. A first electrode 823 secures to a peripheral edgeof a separator 825 on a perimeter to create a first electrode/separatorstructure 827. The structure 827 secures to a peripheral edge of a thirdelectrode 829 on a perimeter to provide a third electrode framestructure 831 in which the electrodes may be positioned in a commonplane. In this embodiment, the first electrode is a cathode. The layersmay be secured to each other chemically by an adhesive, or mechanicallyby a fastener.

Referring to FIG. 9, a process for making a galvanic cell utilizing athird electrode frame structure is shown. A first electrode 923 securesto a separator 925 on a peripheral edge or perimeter to create a firstelectrode/separator structure 927. The first electrode/separatorstructure secures to a third electrode 929 on the outer perimeter toprovide a third electrode frame structure 931. The frame structure hasthe electrodes oriented to be coplanar, or about coplanar. The thirdelectrode frame structure secures to a second separator 933 to provide athird electrode frame/separator structure 935. The third electrodeframe/separator structure secures to a second electrode 937 to providean electrode assembly 939. The second electrode may be positioned tocontact both electrodes of the third electrode frame structure 931through at least one separator. The electrode assembly 939 is assembled941 during molding of a housing structure to provide a galvanicelectrochemical cell 943. The moldable housing may provide structuralsupport and electrical support. Further, the housing defines one or moreapertures through which reagents (such as electrolyte, water and air)may ingress or egress from the housing interior.

One embodiment may include a separator membrane. The membrane may be anelectrically insulating material. In one embodiment, the membrane mayhave a high ion conductivity. According to other embodiments, themembrane may be stable in alkaline environments. Non-limiting examplesof suitable membrane materials include non-woven polyethylene (PE),polypropylene (PP), composites of PE and PP, asbestos or nylon.

In one embodiment, the separator membrane components may besuperhydrophobic membranes. “Super-hydrophobicity,”“super-lipophobicity,” “super-amphiphobicity,” and “super-liquidphobicity” all refer to properties of substances that cause a liquiddrop on their surface to have a contact angle of 150 degrees or greater.Depending upon context, the liquid drop can include, e.g., a water orwater-based drop (super-hydrophobicity), a lipid-based drop(super-lipophobicity), a water-based or lipid-based drop(super-amphiphobicity), or other liquids. Super-liquid phobicitycomprises a generic term indicating a substance that causes a fluid drop(e.g., lipid-based, aqueous-based, or other) to have a greater than 150degrees contact angle.

One embodiment provides a stable environment with substantiallyinvariable humidity. Another embodiment may prevents both dry-out andflooding of fuel cell electrolyte solutions. Yet another embodimentprovides a member and/or assembly for manually adding water in the cell.

In one embodiment, may comprise a nickel metal hydride battery and/orfuel cell technologies. It may utilize metal hydride as an anode and/oran air electrode as cathode so that it has improved energy density, costand environmental impact in comparison to other batteries.

EXAMPLES

Presented below are specific examples of methods for making porous metalhydride anode embodiments. These examples are presented to provideadditional specific embodiments and not to limit embodiments of theinvention.

Example 1 Pellet Formation

A quantity of 10 grams (g) of as-received metal hydride alloy powder ismixed with 4.24 grams of nickel acetate, to form a metal mixture. Themetal hydride alloy powder MH is (AB₅:MMNi_(4.65)Co_(0.88)Mn_(0.45)Al_(0.05)) alloy powder. The metal mixtureis added to 7.12 grams of gel to form a metal gel mixture. The gel ismade by adding polytetrafluoroethylene (PTFE) and carboxymethylcellulose(CMC) into water. Stirring the metal gel mixture at 500 revolutions perminute (RPM) for 30 minutes forms a metal hydride (MH) slurry.

A thin film of the MH slurry is painted onto one surface of a clean 3×3square centimeter plate. The plate is made of foamed nickel. The wetfilm is dried to a dry thin film layer at 80 degrees Celsius for 5 min.Another wet film of the MH slurry is prepared in the same way asdescribed above and painted onto a surface on the other side of the sameNi foam plate. The second wet film is dried at the same conditions asthe first wet film. The steps above are repeated with the slurry wetfilms, until a uniform dry film layer of a determined thickness isformed on both sides of the Ni foam to make a pellet. The pellet is thendried at 120 degrees Celsius in vacuum overnight to form a green pellet.

The green pellet is calcined in a tube furnace. After the calcinations,the porous calcined pellet appears black in color. The process isrepeated to form Samples 1 and 2.

Example 2 Electrode Formation

A metal mixture is prepared and added to a gel as described inEXAMPLE 1. EXAMPLE 2 differs in that rather than 7.12 grams of gel, 5grams of gel are added to form a metal gel mixture. The metal gelmixture is stirred and dried at 80 degrees Celsius. The stirring anddrying processes are repeated until the metal gel mixture is evenlymixed and thoroughly dried.

Half of the dried mixture is added to a 3×3 square centimeters mold. ANi foam plate having the same size as is used in EXAMPLE 1 is then addedinto the mold. The other half of the metal gel mixture is placed on thetop of a Ni foam plate to form a sandwich arrangement. The sandwich maybe pressed under determined conditions as indicated in Table 1. Theprocedure can repeat six times at varying pressures and/or for varyingtimes to form the samples set forth in Table 1. A pressing procedure isas follows:

TABLE 1 Pressing conditions to form the green pellet. Sample Conditions3 2 MPa, 2 min 4 4 MPa, 2 min 5 6 MPa, 2 min 6 8 MPa, 2 min 7 10 MPa, 2min  8 12 MPa, 5 min 

The green pellet may be calcined in a tube furnace. After calcination,the pellet color may be black. The pressed sample may be calcinedaccording to the following procedure:

-   -   1. Heat from room temperature to about 450 degrees Celsius at        about 2 degrees Celsius per minute ramp rate, and maintained at        about 450 degrees Celsius temperature for about 30 minutes.    -   2. Heat from about 450 degrees Celsius to about 500 degrees        Celsius within about 30 minutes, and keep at about 500 degrees        Celsius for about six hours.    -   3. Cool to about room temperature at a ramp rate of about 5        degrees Celsius per minute.

As-prepared MH electrodes are weighed before put into 6 molar (M)potassium hydroxide (KOH) solutions. After soaking for 2 hours, theelectrodes are each weighed to determine how much KOH is absorbed to thesurface and into the pores.

Example 3 System Formation

A working example of the humidity controlling system is set forth inFIG. 2( a). The inside relative humidity (RH) is measured using aprototype with end openings at ambient humidity: about 20 percent, 50percent and 80 percent. A stable internal humidity is obtained.Saturated NaCl solutions provide a stable humidity of about 73 percent.Pure water provides a stable humidity of about 90 percent. The cyclelife for a single cell is measured in RH 45 percent and RH 78 percentrespectively. The results show the higher humidity increases lifetime.

A humidity control system manages water. Humidity controlling materialsinclude saturated organic and/or inorganic salt solution, or polymergels. Means for accommodating such materials include, but are notlimited to, porous ceramics, particle pastes, sponges, and polymer gels.

The embodiments described herein are examples of compositions,structures, systems and methods having elements corresponding to theelements of the invention recited in the claims. This writtendescription enables one of ordinary skill in the art to make and useembodiments having alternative elements that likewise correspond to theelements of the invention recited in the claims. The scope thus includescompositions, structures, systems and methods that do not differ fromthe literal language of the claims, and further includes othercompositions, structures, systems and methods with insubstantialdifferences from the literal language of the claims. While only certainfeatures and embodiments have been illustrated and described herein,many modifications and changes may occur to one of ordinary skill in therelevant art. The appended claims are intended to cover all suchmodifications and changes.

1. An apparatus, comprising: a plurality of electrodes; a housing havingan interior surface defining a volume in which the plurality ofelectrodes are disposed, and an aperture defined by the housing throughwhich at least one air gas stream is in fluid communication with atleast one of the plurality of electrodes; and a humidity-controllingcomponent, wherein the humidity-controlling component is in fluidcommunication with the air gas stream prior to the air gas streamcontacting the at least one of the plurality of electrodes, and thehumidity-controlling component being capable of controlling a relativehumidity of the air gas stream.
 2. The apparatus as defined in claim 1,wherein the humidity-controlling component comprises a saturated aqueoussolution.
 3. The apparatus as defined in claim 1, wherein thehumidity-controlling component comprises a metal salt.
 4. The apparatusas defined in claim 3, wherein the metal salt comprises an alkali earthmetal halide or a rare earth metal halide.
 5. The apparatus as definedin claim 3, wherein the metal salt comprises a metal nitrate, a metalsulfate, or a metal phosphate.
 6. The apparatus as defined in claim 1,wherein the humidity-controlling component is a solution that comprisesone or more salt selected from the group consisting of: lithiumchloride, potassium acetate, magnesium chloride, potassium carbonate,magnesium nitrate, sodium bromide, cobalt chloride, sodium nitrite,strontium chloride, sodium nitrate, sodium chloride, potassium bromide,ammonium sulfate, potassium chloride, strontium nitrate, bariumchloride, potassium nitrate, and potassium sulfate.
 7. The apparatus asdefined in claim 1, wherein the plurality of electrodes comprises ananode, a cathode, and a third electrode disposed within the volume. 8.The apparatus as defined in claim 7, wherein the third electrodefunctions to locate the generation of oxygen spatially distant from theanode during operation.
 9. The apparatus as defined in claim 7, whereina separator membrane is disposed between the aperture and the cathode,wherein the separator membrane allows air to pass into the housing butblocks liquid from flowing out of the housing.
 10. The apparatus asdefined in claim 7, wherein the anode comprises a hydrogen storagematerial.
 11. The apparatus as defined in claim 1, wherein the housingfurther comprises a base operable to hold at least one of the pluralityof electrodes, a tray disposed proximate to the base, the tray definingone or more spaces for containing the at least one humidity-controllingcomponent, and a cover for the tray that is operable to reduce spillageof the humidity-controlling component from the one or more spaces. 12.The apparatus as defined in claim 1, further comprising a ventconfigured to allow at least one air gas stream to exfiltrate thehousing.
 13. The apparatus as defined in claim 1, wherein the humiditycontrolling solution is contained within a material selected from thegroup consisting of a porous particulate substance, a zeolite, a naturalclay, and an inorganic gel.
 14. The apparatus as defined in 1, whereinthe humidity controlling solution is contained within a materialcomprising an organic polymer gel or a porous membrane.
 15. A method,comprising: contacting an intake air gas stream to a humidity buffersolution; adjusting the humidity of the intake air gas stream to anequilibrium humidity level; and providing the humidity-adjusted intakeair gas stream to at least one electrode in a fuel cell.
 16. The methodas defined in claim 15, further comprising maintaining a relativehumidity of the air gas stream within the fuel cell in a range of fromabout 50 percent to about 85 percent.
 17. The method as defined in claim15, further comprising generating hydrogen and storing the hydrogen inan anode capable of storing the hydrogen.
 18. The method as defined inclaim 15, further comprising selecting one or more humidity-controllingcomponent from the group consisting of lithium chloride, potassiumacetate, magnesium chloride, potassium carbonate, magnesium nitrate,sodium bromide, cobalt chloride, sodium nitrite, strontium chloride,sodium nitrate, sodium chloride, potassium bromide, ammonium sulphate,potassium chloride, strontium nitrate, barium chloride, potassiumnitrate, and potassium sulphate.
 19. The method as defined in claim 15,further comprising blocking a flow of liquid water from an air ingress,an air egress, or both an air ingress and air egress of the fuel cell,while allowing the flow of gaseous air.
 20. The method as defined inclaim 19, wherein separating comprises disposing a separator membrane inan air flow path of the intake air gas stream.
 21. An electrochemicalcell, comprising: an air electrode configured to receive an intake airgas stream; and means for maintaining a relative humidity of the intakeair gas stream to be in a range of from about 50 percent to about 85percent.